Indicating lamp and lens assembly for portable electric tools



Oct. 29, 1963 R. HAPPE ETAL INDICATING LAMP AND LENS ASSEMBLY FOR PORTABLE ELECTRIC TOOLS 2 Sheets-Sheet 1 Filed Aug. 29. 1960 IN VEN TORS Reynold Happe,

Norris H. Sui/er and BY Joseph P. Lubas.

L ATT RNEY WITNESS MLQML Oct. 29, 1963 R. HAPPE ETAL 3,109,164

INDICATING LAMP AND LENS ASSEMBLY FOR PORTABLE ELECTRIC TOOLS Filed Aug. 29. 1960 2 Sheets-Sheet 2 INVENTORS. Reynold Happe, Norris H. Sailer and, WITNESS I g 3 BY Joseph P, Lubas.

United States Patent 3,109,164 INDICATING LAMP AND LENS ASSEMBLY FOR PORTABLE ELECTRIC TOOLS Reynold Happe, Princeton, Norris H. Sailer, Metuchen, and Joseph P. Lubas, Linden, NJ., assignors to The Singer Company, a corporation of New Jersey Filed Aug. 29, 1960, Ser. No. 52,694 3 Claims. (Cl. 340-252) This invention relatesuto overload indicators for portable electric tools. I

There has long been a need for a simple but eifective means for indicating externally to the users of electric motor driven portable tools when the internal parts of the motor have reached a. maximum safe operating temperature so that costly burn-outs due to overloading may be prevented. I

For accurate and safe indication it is necessary to place the thermally-responsive unit right at the hot-spot on the motor to be monitored. With the modern compact design of portable tools, it is :difiicult to findv space for mounting on the internal motor parts the usual thermallyresponsive elements such as snap springs or bimetallic devices.

This difliculty is overcome in the present invention by the use of thermistors of miniature size mounted at the hot-spot on the motor to be monitored, for controlling the voltage applied to a gaseous discharge or glow lamp or a signalling device having a critical break-down or ionization voltage. As an example of the small size of the thermistor used in this invention as the thermallyresponsive element, a practical unit is cylindrical in shape with a length of 0.2 inch and a diameter of 0.1 inch.

A resistor placed in shunt with the glow lamp is chosen of such a value with respect to that of the thermistor that the maximum safe hot-spot temperature at the thermistor produces at the glow lamp a voltage equal to its break-down voltage, whereupon the lamp glows to indicate the overload. No make-andbreak contacts required in this circuit which thus removes the maintenance and reliability problems inherent in such contacts.

It is an object of this invention to provide an externally visual overload indicator for portable electric tools which shall be simple, inexpensive, reliable, require minimum space and have no make-andbreak contacts.

It is a further object of this invention to provide simple and effective means for supporting temperature indicating devices in portable electric tools without the use of extra fastening means.

With the above and other objects in view, as will hereinafter appear, the invention comprises the devices, combinations and arrangements of parts hereinafter set forth ,and'illustrated in the accompanying drawings of a preferred embodiment of the invention, from which the several features of the invention and the advantages attained thereby will be readily understood by those skilled in the art.

In the drawings,

FIG. 1 is a vertical elevation view, partly in section, taken through a tool embodying the invention.

FIG. 2 is a detailed perspective view of part of the device of FIG. 1.

FIG. 3 is a wiring diagram showing the electrical relation of the parts.

Referring now to 'FIG. 1, the present invention is illustrated as embodied in a portable belt sander having a cast motor housing in which is seated a stator core 11 of a driving electric motor. The core 11 is shown with the end cover removed and without windings. The usual rotor armature has been removed for clarification.

The motor housing 10 is formed with two spaced lugs Patented Oct. 29, 1963 12-12 as seen best in FIG. 2. Secured to the lugs 12-12 by means of screws 13 is a split handle formed of two hollow halves 14 and 15.

Held clamped in suitable recesses between the handle halves 14 and 15 is a trigger switch 16 and a rubber entrance shield 17 molded onto the line cord .18 for strain relief.

A translucent plastic lens 19 is formed with flanged sides 20 which seat against the top edges of the lugs 12-12 and are held clamped thereon by the handle halves 14 and 15 which are cut away toleave the top surface 21 of the lens 19 exposed for external visual indication.

The top surface 21 forms a smooth flowing contour with the contiguous surfaces of the handle halves 14, 15 and the housing 10.

The lens 19 is formed with an internal cylindrical recess 22 which receives a gaseous discharge glow lamp 23 as shown in FIG. 1. Lip portions 24 formed on the handle halves 14 and 15 retain the lamp 23 within the recess 22 after assembly.

A thermistor 25 is seated against the slot liner 26 in the field coil slot 27 as seen in FIG. 1 and is held in place by the usual impregnated field coil winding (not shown). It is to be understood that this specific posit-ion of the thermistor 25 is merely illustrative and that any position desired to be monitored may be used, the thermistor being small and light enough to impose no difiicult mounting restrictions. It is possible, for example, to secure the thermistor by bending the insulated leads 28, 29 around the core 11.

A fixed resistor 30 of the insulated type is located between lugs 12-12 and is supported by its own leads 31, 32 which connect it in shunt electrically with the glow lamp 23. Lead 33 connects the resistor 30 with one side of the line LL.

FIG. 3 shows the complete wiring diagram. The switch 16 is a three-position switch, having respectively; an off position, a low speed position in which field coils 34, '35, 36 and 37 are all connected in series with the armature 38 across the line LL, and a high speed position in which field coils 34 and 37 only are connected in series with the armature 38 across the line LL.

It will be noted that the thermistor 25 is connected in series with the glow lamp 23 directly across the line L-L. Thus, even with the switch 16 in the off position, the lamp 23 will glow if the maximum safe temperature still exists, which, of course, is desirable.

In operation, as the temperature of the motor at the thermistor rises, the resistance of the thermistor 25 decreases to permit more current to flow through resistor 30 until, when the predetermined maximum safe temperature is reached, the voltage across resistor 30 equals the break-down voltage of the lamp 23 which then glows to indicate the overload. The operator may then refrain from using the tool until the glow lamp is extinguished which indicates that the hot-spot temperature has fallen below the maximum safe value and use of the tool may be safely resumed.

It is important that the indicating device of this invention have a critical break-down or turn-on characteristic so that it does not give any indication until the predetermined maximum safe temperature is reached. This precludes the use of essentially proportional response devices such as filament lamps where partial illumination would be confusing for indicating purposes.

From the above description of the invention, it is clear that the act of assembling the split handle to the frame securely locks the lens 19 in place and no other fastening means is required for its securement. This assembly also retains the glow lamp within the lens.

Having thus described the nature of this invention, What We claim herein is:

1. In a portable electric tool powered by an electric motor, a frame housing said motor, lugs formed on said frame, -a split handle for said frame formed with two mating halves secured to said lugs, a translucent lens formed with a recess and having side flanges in seated engagement with said lugs, said handle halves forming, when assemlbled, means for clamping said lens to said lugs, and a glow lamp seated within said recess and retained therein by lip portions formed on said handle halves. v

2. An indicating lamp and lens assembly fora portable electric tool powered by an electric motor comprising a frame housing said motor, spaced lugs formed on said frame, a splt handle for said frame formed with two mating halves secured to said lugs, a lens formed with an internal recess, external flanges on said lens received in seated engagement with said lugs and clamped to said lugsby the assembly of said mating halves, cut-away portions of said mating halves exposing said lens to external view, and an illuminating device having a critical turn-on voltage fitted within the recess in said lens.

3. An indicating lamp and lens assembly for a portable electric tool comprising a main frame, spaced lugs formed References Cited in the file of this patent.

UNITED STATES PATENTS 854,013 Ayer May 21, 1907 2,165,343 Cole July 11, 1939 2,264,710 Paar Dec. 2, 1941 2,312,099 K-aga-n Feb. 23, 1943 2,411,848 Camras Dec. 3, 1946 2,590,894 Sanborn Apr. 1, 1952 2,715,670 Dicke Aug. 16, 1955 2,769,121 Rogolf Oct. 30, 1956 FOREIGN PATENTS i 7 543,947 Italy May 28, 1956 OTHER REFERENCES QST, Lets Use Neon Bulbs, by McCoy, FIG. 4, page 25, July 1953. 

1. IN A PORTABLE ELECTRIC TOOL POWERED BY AN ELECTRIC MOTOR, A FRAME HOUSING SAID MOTOR, LUGS FORMED ON SAID FRAME, A SPLIT HANDLE FOR SAID FRAME FORMED WITH TWO MATING HALVES SECURED TO SAID LUGS, A TRANSLUCENT LENS FORMED WITH A RECESS AND HAVING SIDE FLANGES IN SEATED ENGAGEMENT WITH SAID LUGS, SAID HANDLE HALVES FORMING, WHEN ASSEMBLED, MEANS FOR CLAMPING SAID LENS TO SAID LUGS, AND A GLOW LAMP SEATED WITHIN SAID RECESS AND RETAINED THEREIN BY LIP PORTIONS FORMED ON SAID HANDLE HALVES. 